The invention relates to an additive for soluble oil, intended to increase the lubricity of this oil, especially when it is applied as a thin and xe2x80x9cdryxe2x80x9d film to a steel surface.
This type of soluble oil is used in metal-working operations, as described for example in patent U.S. Pat. No. 4,027,512.
To prepare the operations of forming bare steel sheet, for example by drawing, it is known to carry out an oiling treatment on the surface of this sheet intended to come into contact with the surfaces of the forming tools.
This oiling treatment is intended to provide good lubrication between the sheet surface and the surface of the tools under the forming conditions.
Applied to bare steel sheet, this oiling treatment often has another function, of temporarily providing corrosion protection, as described in patent EP 0 577 486 (Sollac).
To carry out xe2x80x9coilingxe2x80x9d-type surface treatments, it is known to use:
either oils called xe2x80x9cwholexe2x80x9d oils, that is to say those being in the form of an oily homogeneous liquid phase;
or oils called xe2x80x9csolublexe2x80x9d oils, that is to say those which can be emulsified in water and applied as an emulsion, the emulsion having at least two different phasesxe2x80x94an aqueous liquid phase and an oily phase dispersed in the liquid phase.
An advantage of soluble oils over whole oils is that they make it possible to obtain more easily homogeneous coatings which are very thin or have a very low surface density, for example less than or equal to 1 g/m2.
This is because, unlike whole oils, soluble oils may be easily applied by spraying. This makes it possible to ensure that there is a homogeneous coating of very small thickness on the surface to be treated.
The abovementioned patent EP 0 577 486 describes a method of treating the surface of a metal workpiece using an oil emulsion; according to the invention forming the subject matter of that patent, this treatment is suitable for forming, on this surface, a thin homogeneous and protective film, of dry appearance, with a surface density of less than 1 g/m2.
The abovementioned patent U.S. Pat. No. 4,027,512 describes the use of fatty acids as additives for increasing the lubricity of soluble oils for the working of metals and the method of incorporating these additives into emulsions.
Among the advantages associated with the application of very thin oil films to metal surfaces, mention may be made of:
the ease of handling these workpieces, because they do not have a xe2x80x9cgreasyxe2x80x9d appearance;
the elimination, in certain cases, of a cleaning step prior to subsequent conversion operations such as, for example enameling or painting;
on the other hand, should subsequent cleaning be necessary, the cleaning is easy since the amounts of oil to be removed are smaller;
the possibility of obtaining effective temporary corrosion protection, especially on bare steel sheet, as described in the abovementioned patent EP 0 577 486.
Unfortunately, the lubricating properties of the thin films obtained from an oil-in-water emulsion are generally inferior to those of thicker films obtained by applying whole oils; this is because, on a thin film:
problems of chattering often arise;
the friction coefficient and/or the maximum drawing force (as defined in the xe2x80x9cMETHODSxe2x80x9d section below) are too high.
It is an object of the invention to propose an additive for increasing the lubricity of soluble oils when they are applied to bare steel surfaces, especially when they are applied as a thin dry film to these surfaces.
For this purpose, the subject of the invention is an oil-in-water emulsion intended for a lubrication treatment of a steel surface, containing at least one lubrication additive chosen from the group comprising the products corresponding to the following formulae:
(R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94PSSxe2x80x94(CH2)nxe2x80x94COOxe2x80x94(CH2)mxe2x80x94CH3 with n and m chosen so that n+mxe2x89xa722;
(R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94PSSxe2x80x94CH2-"PHgr"-OH, or hydroxybenzyl O,Oxe2x80x2-dialkyl dithiophosphates;
(R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94PSSxe2x80x94CH2-"PHgr"-Oxe2x80x94(CH2)nxe2x80x2xe2x80x94COOxe2x80x94(CH2)mxe2x80x2xe2x80x94CH3 with nxe2x80x2 and mxe2x80x2 having any value,
where PSS denotes the dithiophosphate functional group, (R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94 denotes O,Oxe2x80x2-dialkyl terminal groups, xe2x80x94CH2-"PHgr"-OH denotes the hydroxybenzyl functional group and xe2x80x94CH2-"PHgr"-Oxe2x80x94 the benzyloxy functional group.
The invention also has one or more of the following characteristics:
if the additive satisfies the formula (R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94PSSxe2x80x94(CH2)nxe2x80x94COOxe2x80x94(CH2)mxe2x80x94CH3, n and m are each greater than or equal to 3;
if the additive satisfies the formula (R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94PSSxe2x80x94CH2-"PHgr"-Oxe2x80x94(CH2)nxe2x80x2xe2x80x94COOxe2x80x94(CH2)mxe2x80x2xe2x80x94CH3, nxe2x80x2 and mxe2x80x2 are each greater than or equal to 3;
if the additive satisfies the formula (R1,2xe2x80x94Oxe2x80x94)1,2xe2x80x94PSSxe2x80x94CH2-"PHgr"-OH, no other polar group than the xe2x80x94OH functional group is grafted to the aromatic ring "PHgr" of the hydroxybenzyl functional group.
Preferably, the proportion of lubrication additive in the oily phase of the emulsion is between 2% and 6%.
The subject of the invention is also a treatment method for the lubrication of a steel surface, comprising the steps of applying this emulsion to this surface followed by a drying step suitable for forming a dry film on said surface, the dry film preferably having a surface density of less than or equal to 1 g/m2.
The subject of the invention is also the use of this treatment method for lubricating a steel sheet before it is formed, since the oiled sheet with a dry appearance obtained by this treatment method does not need to be reoiled before the forming operation, especially by drawing.
If the lubrication treatment method according to the invention is applied to a sheet intended to be enameled or to be painted, the invention advantageously avoids having to carry out any cleaning operation after this lubrication and prior to this actual enameling or painting, especially if the dry film which results from this treatment has a surface density of less than or equal to 1 g/m2, and even if a forming operation is interposed between the lubrication treatment and the enameling or painting operation.